It is accepted practice for a standing musician to rest the back of an acoustic guitar against his torso while performing. The area of contact with the back of the guitar may be large or small depending upon the stature and playing technique of the guitarist. Regardless, it has been found that any contact of this sort tends to reduce the resonance of the guitar and degrade its tone.
Holding the guitar away from the torso while performing for a prolonged period of time leads to fatigue and, ultimately, poor musicianship. Some have proposed stands and holders of various sorts to reduce such fatigue. As a group, these devices have been difficult to use and, for this reason, have not seen widespread acceptance among serious musicians.